A layer having a composition different from that of an inner part of glass is liable to appear on the surface of multi-component oxide glass due to the influences of water vapor and heat in the atmosphere.
For example, one of the productions of a porous glass material is to utilize a phase separation. A phase separation is generally induced by heat treatment of holding molded borosilicate glass at high temperature for a long period of time. By etching the glass with an acid solution to elute a silica-poor phase, the porous glass is obtained. A skeleton of the porous glass is mainly formed of silicon oxide. For making the porous glass material utilizing a phase separation, borosilicate glass mainly containing silicon oxide, boron oxide, and an alkali metal oxide is generally used.
However, for example, as also described in NPL 1, in a phase separation induced by holding glass at high temperature for a long period of time (hereinafter, referred to as “phase separation heating treatment”) and phase separation, boron or the like on the surface of glass is partly lost to cause a composition displacement, and thus, a denatured layer is formed on the surface of the glass. The denatured layer causes a problem of inhibiting etching with an acid solution in a later process. Hence, the denatured layer is required to be removed.
For example, PTL 1 discloses a technique of removing at least 10 μm of outermost surface glass by polishing. However, with the removal by polishing, glass needs to have mechanical strength to some degree, and there may be problems such as roughening of the surface by polishing and the remaining of an abrasive in performing later processes in some cases.
PTL 2 also describes that an outermost surface glass layer of borosilicate glass is removed by a chemical method using hydrofluoric acid, ammonium hydrogen difluoride, or an alkaline glass corrosive.
PTL 3 describes a method of removing a denatured layer by a chemical procedure involving washing a denatured layer on the surface of glass with an alkaline aqueous solution. However, PTL 2 and PTL 3 do not disclose a method of selectively removing a denatured layer. Further, in the case where a denatured layer is formed on thin glass or a glass thin film, it is necessary to selectively remove only a denatured layer, which requires a new facility.